Recently, activated polyethylene glycols have been widely used as important carriers for drug delivery systems. As such activated polyethylene glycols for the purpose of the pharmaceutical uses, those containing little impurities have been required from the viewpoints of performance and safety of drugs to be produced by modifying them. A polyethylene glycol having an amino group is itself a modifier of drugs and also is a raw material for synthesizing other activated polyethylene glycols. When a polyethylene glycol having no amino group is contained as an impurity in a polyethylene glycol having one amino group, in the case where an activated polyethylene glycol is synthesized using it as a raw material, a highly pure activated polyethylene glycol having one amino group cannot be synthesized. Therefore, a modification ratio decreases when the product is bound to a drug, and there is a possibility that expected effects cannot be obtained. Since the polyethylene glycol compound having no amino group that is an impurity and the polyethylene glycol having one amino group are high-molecular-weight compounds, physicochemical properties are resemble, so that separation from each other is difficult by a common technique.
On the other hand, the purification of the drug after bonding to the polyethylene glycol also has a technical problem that separation thereof is difficult. In the case where the purification is carried out, a drug yield is remarkably decreased and hence a production cost increases. Accordingly, it is desirable to remove the polyethylene glycol compound that is an impurity as far as possible prior to the bonding to the drug.
In general, in the case where impurities such as low-molecular compounds and salts are removed from macromolecular compounds, a crystallization operation and an extraction operation are frequently performed. In the case where the impurity to be removed is a low molecule, physicochemical properties remarkably vary depending on large difference in molecular weight and particularly, since a large difference in solubility takes place, separation and purification become possible. On the other hand, in the case where the molecular weight of the impurity is large, not so large difference in physicochemical properties such as solubility does not appear. Therefore, it is difficult to separate and remove the polyethylene glycol compound having no amino group from the polyethylene glycol compound having one amino group by a common operation for removing a low-molecular impurity.
On the other hand, with regard to a low-molecular-weight compound having an amino group, when it is protonated at low pH, hydrophilicity increases owing to ionic interaction. Therefore, in the case where the hydrophilicity of an impurity is low, it is possible to extract the impurity into an organic layer to remove it by an extraction operation using a suitable organic solvent. However, since a polyethylene glycol compound itself has high hydrophilicity, it is difficult to separate the polyethylene glycol compound having one amino group and the polyethylene glycol compound having no amino group that is an impurity by extraction with an aqueous layer and a single hydrophobic organic layer. On the other hand, when an organic solvent such as a halogenated hydrocarbon that is a good solvent for a polyethylene glycol compound is used, the polyethylene glycol compound is extracted into an organic layer irrespective of the existence of an amino group, so that separation and purification is also impossible.
Here, the characteristic features of conventional techniques are described and also it is describe whether the purification of the polyethylene glycol compound having one amino group is possible by the techniques and whether the techniques can be carried out in an industrial scale.
As one method, as shown in Patent Document 1 (JP-A-8-165343), in the case of a polyethylene glycol having an amine, it is possible to separate it by a chromatogram using an ion-exchange resin. However, since such a purification method using an ion-exchange resin is a method of principally utilizing interaction and adsorption phenomenon to a solid surface, it is necessary to perform a purification treatment using a large amount of the resin under a dilute solution condition. Since the step has to be performed under such dilution that the concentration of the polyethylene glycol compound in the step is about 1 to 2% in order to suppress a decrease in separation ability, so that the method is not sufficiently satisfactory from the viewpoint of industrial productivity. Moreover, finally, a waste of a large amount of the ion-exchange resin is discharged and thus this method is also a purification method having a problem on industrial utilization.
On the other hand, as shown in Patent Document 2 (JP-A-2010-254978), there is a method in which a polyethylene glycol compound that is an impurity is physically adsorbed on a adsorbent and removed. When a suitable adsorbent is used, a polyethylene glycol impurity having two hydroxyl groups that is an impurity is preferentially adsorbed but there is no description that an impurity can be selectively removed depending on the presence or absence of an amino group. Furthermore, in the case of performing the method in an industrial scale, there is a problem that a large amount of the adsorbent filtrated off after the adsorption treatment becomes waste.
On the other hand, there is a method in which a high-molecular-weight polyethylene glycol is mixed with an organic solvent in water to fractionate and remove an impurity having different molecular weight, as shown in Patent Document 3 (JP-A-2010-254981). It becomes possible to remove the impurity having different molecular weight by controlling salt concentration of the aqueous solution and extraction temperature. Since the method is a simple method, it is possible to carry out the method in an industrial scale and there is a large advantage that waste such as an adsorbent is not discharged. However, by this method, polyethylene glycol compounds having the same molecular weight cannot be separated from each other depending on the presence or absence of an amino group.
On the other hand, as shown in Patent Document 4 (US2009/000834), there is a method of separating a low-molecular compound having two tertiary amines from a low-molecular compound having one tertiary amine in the structure. An aqueous solution of low-molecular compounds having tertiary amine(s) are adjusted to pH 4 with an acid to protonate the amine group. When ethyl acetate is mixed thereto to cause layer separation, the objective low-molecular compound having one tertiary amine can be selectively extracted into the ethyl acetate layer. Moreover, as shown in Patent Document 5 (WO2006/057778), there is a method of separating Methamphetamine that is a secondary amine from Benzphetamine that is a tertiary amine. A roughly purified product of Benzphetamine is dissolved in water and the solution is adjusted to pH 6 with an acid to protonate the amino group. When toluene is mixed thereto to cause layer separation, the objective Benzphetamine can be selectively extracted into the toluene layer. In both methods, the separation of the objective substance from the impurity is made possible by adjusting the aqueous solution to suitable pH by adding an acid. These methods include relatively simple steps and discharge little waste and hence the methods are sufficiently applicable even in an industrial scale. However, since a polyethylene glycol compound has high hydrophilicity, the compound cannot be extracted into an organic layer with such ethyl acetate or toluene as used in the above system, irrespective of the presence or absence of the amino group. On the other hand, in the case where an organic solvent such as a halogenated hydrocarbon-based one is used, even when pH is lowered to protonate the amino group and thus the hydrophilicity is enhanced, the polyethylene glycol compound having one amino group and the polyethylene glycol compound having no amino group are both extracted into the organic layer, so that selectivity is not realized. Accordingly, extraction purification cannot be achieved by applying these methods.
The polyethylene glycol compounds having one amino group for use in pharmaceutical uses all contain polyethylene glycol impurities formed during the production process. These impurities are polyethylene glycol compounds having about the same molecular weight as that of the objective substance and hence removal thereof includes many problems.